One might postulate a document feeder comprising a feed wheel urging the foremost documents in a first direction, while also using a separator wheel W to cooperate therewith by restraining all following documents (e.g. rotating counter to W to drive documents oppositely) so that only a single document is fed at any one time.
It would often be preferable to make wheel W of large radius to be able to thrust the foremost document along a relatively straight path, a path that is along the document's original stacked-plane or close to it (e.g. note tangent to wheel tends to become so as the wheel radius increases).
If two documents are in the feedwheel/separator nip, there will invariably be some overlap. Hopefully this overlap will be small enough so that the interdocument space creating device can correct it.
To avoid any overlap at all, the feedwheel and separator should work together to prevent more than one document from entering the feedwheel/separator nip. I am not aware that any workers have addressed this issue. Picture a simple model was formulated to investigate the effects of two documents entering the feedwheel/separator nip. FIG. A-1 illustrates this, with sheet stack ST, and foremost sheet d-1, and following sheet d-2. Feed wheel W will be understood as urging d-1 in the "Go" direction (arrow) while non-rotating separator wheel roll R urges d-2 not to follow d-1 (see force F on R)
As a "first-order" approximation, nudger friction force is neglected here, and it is assumed that the documents d-1, d-2 are just slightly in to the nip between W and R, while feedwheel W is trying to drive both documents the rest of the way into the nip.
For this situation, two parameters are calculated. Each is the ratio of the sliding friction force to the non-sliding tangential force between the document and the component. They are called "feed margin" and "separator margin". The higher their values, the more margin there is for successful feeding (allowing sheet d-1 to move into the nip) and separation (preventing sheet d-2 from moving into the nip).
Typical input parameters are suggested in Table I; they might be (all units in the inch-pound-second system):
TABLE I feeder radius: RF = 1.000 separator radius: RS = 1.000 doc1 thickness: T1 = .004 doc2 thickness: T2 = .004 normal force: NS = 4.000 force sharing: C = 1.000 feed/doc1 fric coef: CFF1 = 1.000 sep/doc2 fric coef: CFS2 = .800 doc1/doc2 fric coef: CF12 = .400
The results for several runs changing feedwheel and separator radii, areas follows in Table II:
TABLE II change to above data feed margin separator margin None 1.969 2.671 RF = 2.000 2.168 2.812 RF = 4.000 2.309 2.922 RF = 8.000 2.396 2.993 RS = .500 2.424 3.815 RS = .250 2.445 6.087
Conclusion: Having a large feedwheel radius and/or a small separator-roll radius improves margins for feeding and especially for separating.
FIG. A-2 shows a feeder used to verify the foregoing. Here, a document stack ST is served by a nudge-roll 1-N periodically urging the foremost document into the nip between a feedwheel 1-F and a separator belt SB driven by rollers 1-S and 1-R, to advance this document in direction (Go) of arrow. Here, separator belt velocity is kept relatively low, to greatly favor separation. Here, the velocity of leading edges is slightly higher than the separator belt velocity, giving greater probability to producing "feedchecks". I find that one solves the feedcheck problem by moving the entire separator mechanism toward the front of the machine as far as possible without having to change adjacent parts. This effectively decreases the separator radius due to a subtle position relationship between the feedwheel and the separator mechanism.
FIG. A-2 illustrates this subtle relationship. The feedwheel 1-F contacts the separator belt SB between two rollers. Therefore, the effective radius of the separator near the nip is larger than the roller radius near the nip. The actual value of this radius is a complex function of the elastic properties of the separator belt, the forces applied by the separator mechanism springs, and geometry of the separator mechanism. However, moving the entire separator mechanism to the right (towards the feed-direction) reduces this effective radius at the nip. This is precisely what one should do to improve the feedcheck rate.
Ideally, minimal separator radius is obtained in FIG. A-2 when the entire separator mechanism is moved far enough to the right so that, when a line is drawn through the centerlines of the feedwheel and the belt separator belt drive roller 1-S, the nip lies on this line. This is additional evidence confirming the conclusion arrived at from FIG. A-1.
The reasons for preferring minimal separator radius and maximal feedwheel radius may be argued intuitively. Ideally, the separator should present as blunt a surface as possible to the second and subsequent documents in the stack. This means a "zero radius" object: a rectangular block, would be ideal, though it would be difficult to implement a moving separator under these conditions. The feedwheel wants to be a large radius so as to minimize the vertical component of force that drives against the separator, leaving all the available force for feeding the document. Ideally, a flat (infinite radius) "feedwheel" is desired.
Consequences of Optimal Geometry Conditions
Having a "small-radius" separator is also inimical to long service life. To achieve long separator service life, a belt should be used for the separator to increase surface area, while still being able to maintain small separator radius near the feedwheel/separator nip.
Having a large feedwheel radius is conducive to extended service life. However, large feedwheel radius results in larger drive torque, requiring larger motors. Also, large feedwheel radius results in much, much larger feedwheel inertia. With the trend now towards quickly accelerating and decelerating feedwheels to achieve "constant-separation" feed, interdocument space correction, smart feeders, etc., it might seem that large radius feedwheels are not desirable; or that. Feed margins should be achieved by other means, such as higher feedwheel/document friction coefficient, and driving the nudger faster than the feedwheel.
Normal Forces:
The process for specifying normal forces is rather complex. The basic process that I contemplate is illustrated in FIG. A-3.
As a first order approximation, the value of the separator normal force (e.g. F.sub.1, FIG. A-1) does not affect feeding and separation. However, experimental friction measurements with currently used separators and feed tires suggest that the separator normal force should be kept small to increase friction coefficients. Small separator normal forces should also improve the service life of the feed tire and separator, provided there is enough wear to remove contaminants.
But, small separator normal force may also negatively affect the interdocument space creation process. FIG. A-4 illustrates this (e.g. in an array used for high-speed and medium speed sorting).
In FIG. A-4, after document d-1 leaves the feeder, a higher speed, accelerator roller DR accelerates d-1 to a higher speed via the friction force between roller DR and the document. While this acceleration is taking place, and while the next document d-2 is still in the feedwheel/separator nip, an interdocument space (between d-1 and d-2) is created because document d-l has now been accelerated is at a greater velocity. Interdocument space continues to increase until document d-2 leaves the feedwheel/separator nip and is accelerated to the same speed as document d-1.
If the drive force of the accelerator roller DR is greater than the slip force at the feedwheel/separator nip, the accelerator roller DR will pull the document out of the feeder. This could have negative effects on the interdocument space consistency and it can cause excessive wear of the feed tire and separator.
Although the accelerator roller DR is the first roller after the feed wheel (e.g. as in some high and medium speed sorters) it does not need to be the "first". If the interdocument space creation process were placed further downstream, of the feed wheel, it would not be a factor in feeder design.
Complex Model
As artisans will agree, such a feeder is a complex system. A complete analysis is very complex. Because of the statistical nature of friction and document lengths, Monte Carlo simulation is required to perform the analysis. Also, there are many different kinds of document positions that must be considered: two documents in the nip, one document approaching the nip, two documents approaching the nip, etc. A related computer program analysis will contain many "if-then-else" routines.
Novel Feature:
As a salient feature hereof, this invention presents an "effectively-large-radius" (or "pseudo-large" radius) feedwheel in the area of a feedwheel/separator nip, without having to actually use a complete (full circle) large radius feedwheel. In this way, it can improve document feeding reliability without requiring the space, weight, etc. of a complete large radius feedwheel.
What is New or Different:
With such a feature, a feeder belt (e.g. as in FIG. 1) can be provided, to slide over a low friction, large radius block, or the like--as a sheet-feeder, so that the "large radius" is effectively obtained locally at the feedwheel/separator nip. This gives the advantages of large radius feedwheel in a small space and without large inertia.
Thus, it is an object hereof the address (at least some of the aforementioned problems, and to provide the hereincited, and related, advantages and functions. A related object is to provide such in an automatic, large-radius feeder unit of a sheet-sorting machine.
The methods and means discussed herein, will generally be understood as constructed and operating as presently known in the art, except where otherwise specified; with all materials, methods and devices and apparatus herein understood as implemented by known expedients according to present good practice.